The present invention relates to a method for growing a polycrystal or single crystal of a compound semiconductor by using the horizontal crystal growing process.
The horizontal Bridgman method (HB method) and temperature gradient freezing method (GF method) are typical examples of conventional methods for horizontally growing the crystal of a compound semiconductor. In either case, a volatile element (e.g., group V element) is provided on the one end side of a quartz ampule, while a boat containing a fusion element (e.g., group III element) is located on the other end side. The ampule is put horizontally into an electric furnace, which is kept within a predetermined temperature profile. By doing this, the element evaporated at the one end or the low-temperature side of the ampule is dissolved to saturation in the element fused at the other end or the high-temperature side. Thus, a molten compound semiconductor is prepared in the boat. The temperature profile has a temperature-gradient section which includes a melting point MP of the compound semiconductor (freezing point of the melt) between the low- and high-temperature sides.
According to the HB method, the ampule is moved relatively to the electric furnace in a gradual manner, from the high-temperature side to the low-temperature side, whereby the crystal is grown from the one end of the boat toward the other end thereof. According to the GF method, on the other hand, the crystal is grown from the one end of the boat toward the other end thereof by controlling a heater of the electric furnace so that the temperature profile is lowered gradually, starting at the one end side of the high-temperature portion and ending at the other end side thereof.
Meanwhile, in growing a polycrystal by the crystal growing methods described above, the following problems arise depending on the growing conditions. In the case of a polycrystal, the composition of intergranular substances become uneven due to precipitation of impurities or the like. Thus, the more the intergranular substances, that is, the smaller the grain size, the less stable the general composition will be. In order to obtain a polycrystal of a uniform composition, therefore, it is necessary to produce one with a large grain size. No seed is used, however, in growing a polycrystal. If the temperature gradient at a crystal growth starting end is too small, therefore, a crystalline nucleus cannot be easily formed, so that supercooling is liable to be caused. If supercooling takes place, freezing starts suddenly when the temperature is lowered by some degrees from the melting point. In consequence, only a polycrystal of a small grain size can be produced. If the temperature gradient at the crystal growth starting end is made too large, on the other hand, the speed of crystal growth increases. Also in this case, therefore, the grain size of the polycrystal obtained is inevitably small. In forming a polycrystal of InP, for example, inclusions of indium or voids may possibly be caused.
In growing a single crystal of a compound semiconductor in a boat by the conventional horizontal Bridgman method or gradient freezing method, on the other hand, a seed crystal is provided at the front end of the boat, and a melt of the semiconductor in the boat is caused to adhere to the seed crystal. Thereafter, a single crystal is grown from the seed crystal.
If the wettability between the seed and the melt is poor, however, an undesired additional crystalline nucleus will be produced separately, and develop into a polycrystal. Depending on the type of the compound semiconductor (e.g., group II-VI compound semiconductors, such as ZnSe, ZnS, CdTe, ZnTe, HgTe, CdHgTe, etc., and group III-V mixed crystals, such as GaAsP, InGaP, GaAlAs, AlInP, InGaAsP, etc.), moreover, the seed crystal may not be able to be easily obtained. Also, seeding may sometimes be difficult due to problems related to equipment. Conventionally, therefore, a method for growing a single crystal without using any seed crystal has been expected to be developed. However, no such method has hitherto been proposed.
If a crystal is grown with use of a high-cost seed crystal, such as InP, furthermore, the resulting crystal is inevitably high in cost. Thus, if a single crystal can be produced without using any seed crystal, then the cost of the single crystal will be reduced highly effectively.